SLS Lecture 3 : I/O, Process Control and Credentials

Contents

SLS Lecture 3 : I/O, Process Control and Credentials¶

Running and Managing Programs (Commands)

Processes, Files and Streams¶

What is a process in more detail and what can it do
- a running program
  - process can launch another processes
    - bash is implemented to start commands as process (when needed)
    - passes command line arguments and environment variables
- Processes, Files and channel/stream model of I/O
  - files and Kernel objects
    - read, write
    - everything is a file
  - Streams: Processes and Files
    - open : attach a file as a stream
    - file descriptors/handles : stream
      - read and write bytes to a stream
      - close
      - dup
    - standard input, standard output, standard error
  - Shell Syntax:
    - echo 'Hello world' > hello
    - cat hello
    - cat < ./hello
- We now can understand what a pipe is
  - pipe file object
  - Process 1 stdout into pipe and Process 2 stdin from pipe
  - Shell Syntax:
    - ‘ls -1 | wc -l’
    - ‘ls -1 | grep ‘^l*’ | wc -l’
    - mknod mypipe p and mkfifo mypipe

Credentials and file permissions¶

Process have id’s associated with them
- a single user id : id
  - a single number that maps to a string user name (/etc/passwd)
- set of group ids
  - user has a primary group but can be in many secondary groups
  - each has a number that maps to a name (/etc/group)
  - each group can have many users
- ps auxgww
  - process’s inherit their ids from their parent
Files have id’s and permissions ls -l, chmod, chown, and chgrp
- user, group, other -> read, write, execute
- kernel ensures that process id’s and requested operations match permissions
Process management
- ps - look at all processes
- The shell and its children
  - & : foreground and background
  - ctrl-z
  - jobs
- so we know how to start process, list them how about stopping
- kill
  - signals
  - ctrl-c
  - or without prejudice